This is the unexpurgated, pre-edited version of the article
"Improved Anode-Circuit Parasitic-Suppression For Modern
Amplifier-Tubes" that appeared on page 36 in the October 1988 issue
of QST. A more recent treatment of the subject appeared in the
September and October 1990 issues of QST. The article is titled
"Parasitics Revisited. "

The purpose in publishing this manuscript is to allow the reader to
see whether on not QST is influenced by advertisers. To do this, open
up a copy of the Oct. 1988 QST, and compare. Anything in parenthesis
is not part of the manuscript text. The information in parenthesis
was added later.

--------------------------------------------------------

Improved Anode-Circuit Parasitic-Suppression
For Modern Amplifier-Tubes

The traditional copper-inductor/carbon-resistor anode
[plate] parasitic-suppressor has been used in vacuum-tube
amplifiers for at least 50 years. The earliest record of an anode
parasitic-suppressor that I can locate was in a transmitter that was
built in the early 1930s by the (Art) Collins Radio Company.

(In late 1990, I was made aware of some interesting information on
anode-circuit VHF parasitic suppressors in the 1926 Edition of The
Radio Amateur's Handbook. This information was inexplicably omitted
from post-1929 editions. Info provided by Dave Newkirk, WJ1Z)

Much of the reason for Art Collins' early success can be attributed
to the fact that he, almost alone, understood that where RF is
concerned there is no such thing as a zero-potential "ground" and
that any wire or strap was a capacitor-inductor VHF tuned-circuit as
well as a conductor. He understood that an "RF-choke" acted like a
short-circuit at certain frequencies and that sometimes a resistor
would make a better RF-choke than an RF-choke! Because he understood
these "RF secrets", he was the first manufacturer to build a
transmitter that: worked on all frequencies up to 14.5MHz, was stable
and could be tuned up every time with no surprises.

Anode parasitic-suppressor design has not changed during the last 50+
years while vacuum-tube design has changed markedly. In the 1930s,
40s and 50s, a "high-Mu triode" had a (voltage) amplification factor
of 40. Today, a "high-Mu triode" usually indicates an amplification
factor of 100 to 240. A fifty+ year-old parasitic-suppressor design
that was usually successful at preventing oscillation in an
amplifier-tube with an amplification of 40, may not be as successful
on a modern amplifier-tube that has much more gain.

Modern amplifier-tubes have another factor, in addition to higher
voltage gain, that makes the job of the traditional inductor/resistor
VHF parasitic-suppressor more difficult. That factor is higher
frequency capability. Ancient amplifier-tubes could barely be coaxed
into amplifying at 28MHz. The 203A that was used successfully in the
Collins 150B transmitter had a full-power rating of 15MHz.

Modern Amplifier-Tube Performance:

The popular 8802/3-500Z triode has an average amplification factor of
130 (Eimac) to 200 (Amperex). The Amperex version appears to be
electrically equivalent to the 8163/3-400Z with the exception of the
anode dissipation rating. The maximum-input rating of the Eimac
3-500Z, for "radio frequency power amplifier or oscillator service"
is 110MHz. 3-500Zs work well above 110MHz if the power is de-rated as
frequency increases. Other types of modern amplifier-tubes commonly
used in HF-amplifiers have an even higher amplification factor and a
frequency rating of up to 500MHz. The 8874 is a good example of a
high gain, 500MHz triode. It has an average amplification factor of
240! This is definitely a high-Mu triode.

Oscillators:

If an amplifier-tube can amplify at a frequency, it can usually be
made to oscillate at that frequency. This is good news for oscillator
builders and bad news for unwary amplifier builders.

In addition to frequency capability, there are some other
prerequisites that must be met before oscillation can be achieved: a
feedback path between the output and the input of the amplifier and
high-"Q" resonant circuits in the output lead and in the input lead
to the amplifier-tube that are resonant near the same frequency. The
resonant circuits are essential because they act like a flywheel and
sustain the oscillation during the portion of the cycle that the
amplifier-tube is not conducting and amplifying.

The (Incomplete) Schematic Diagram:
Understanding the nature of the parasitic-oscillation problem would
be much easier if the schematic diagram of an amplifier circuit would
show the interconnecting input and output leads to the amplifier-tube
for what they actually are: inductors. These incognito inductors,
combined with the inter-electrode capacitances of the amplifier-tube,
form unavoidable VHF self-resonant circuits. See Figure
1, A-B-C The typical frequency range of these resonances is from
90MHz to 160MHz in 1500W HF amplifiers.

The Parasitic-Oscillation Seed-Voltage:
The essential question is: Where does the initial VHF voltage come
from that starts the self-resonant flywheels in motion that causes
the parasitic-oscillation to take place? Certainly, it can not come
from the exciter because all exciters have a built-in low-pass filter
that is very effective at blocking any VHF signal. This leaves only
the amplifier as the source of the seed-voltage.

The answer to that pivotal question involves Q. Q represents the
"Quality" of a tuned circuit component. More Quality should be
better. An old adage says: "more is not always better". Where
amplifier design is concerned, more Q is certainly not always better.
The appropriate Q for each part of the circuit is the best design.
For example: HF tank-circuit components should have a high-Q. and, as
I will explain, anode leads should have a low-Q.

For the purpose of this discussion, the most important rule about Q
is: The RF-voltage that is developed across a resonant circuit is
directly proportional to the Q of the resonant circuit.

This principle is best illustrated by the antique spark-transmitter.
In a spark-transmitter, the transient-currents from a motor-driven
rotary spark-gap (a motorized switch) were passed through a high-Q
tuned-circuit. This caused the tuned-circuit to "ring" at its
resonant frequency which produced a surprising amount of RF voltage
and power. The tuned-circuit acts like a flywheel after each impulse.
It coasts a bit after each impulse and then stops, like the ringing
of a bell. This is referred to as "flywheel-effect". Lowering the Q
will reduce the flywheel-effect.

Amplifiers are routinely subjected to numerous turn-on, switching,
keying, and voice transient-currents. These transient-currents pass
through the VHF self-resonant anode-circuit and the VHF self-resonant
input-circuit. Each transient-current causes the input and output
self-resonant circuits to ring and generate an invisible, damped-wave
VHF voltage that is proportional to the VHF-Q of these circuits This
is the source of the VHF seed-voltage that initiates the
parasitic-oscillation.

Part of this seed-voltage will be fed back to the input of the
amplifier by the feedthrough/feedback capacitance inside the
amplifier-tube. The VHF voltage will then be amplified by the
amplifier-tube and it will appear in the anode-circuit where some of
it will be returned to the input of the amplifier-tube by way of the
feedback-capacitance.

If the amplified VHF voltage arrives with the right phase and
amplitude, an even larger signal may be fed back to the input of the
amplifier. When this occurs, the parasitic-oscillation is off and
running. This would not be a problem if the considerable energy that
is generated by the VHF parasitic-oscillation could be safely
dissipated in the load that is connected to the amplifier.
Unfortunately, the VHF energy can not reach the output connector of
the amplifier because it can not pass through the HF tank-circuit
inductor. This inductor acts as an RF choke to the VHF energy. This
traps the VHF energy in the anode-circuit. With no load, the
grid-current and grid-dissipation of a high-Mu triode oscillator
becomes excessive in a matter of milliseconds. This can start a chain
reaction of events that almost simultaneously results in a loud bang
and can cause severe damage to the amplifier.

Grounded-grid Oscillators:
Making a grounded-grid amplifier oscillate is easier than it might
seem: In a grid-driven, grounded-cathode amplifier, the output and
input voltages are 180 degrees out of phase. They oppose each other.
Before regeneration can occur, the output and input voltages must be
made in-phase, to aid each other, by adding a phase-shift circuit. In
a grounded-grid amplifier the output and input voltages are already
in-phase and aiding each other.

For many years it was assumed that grounded-grid amplifiers were
inherently stable because the "grounded"-grid acts as a shield
between the input and the output circuits, thereby blocking
regeneration and oscillation. At HF this logic is true but at VHF,
the logic is false because no matter how carefully an amplifier-tube
is designed, at some frequency the "grounded"-grid will become
self-resonant. This is due to the unavoidable, combined inductances
of: the grid structure, the internal leads, external leads, and the
tube socket, resonating with the capacitance of the grid structure.
In a 3-500Z triode, the directly (as is possible) grounded-grid will
self-resonate at about 95MHz. As frequency increases above grid
self-resonance, the grid exhibits inductive reactance, and the grid
is no longer "grounded".

When the grid is not truly grounded, as is the case above its
self-resonant frequency, the assumption about the shield, that we are
depending on to block regeneration, is in serious trouble. And, to
make matters worse, as the frequency increases into the VHF region,
the feedthrough capacitance from the input [cathode] to the
output [anode] of the amplifier has fewer and fewer ohms of
capacitive reactance.

In other words, As the frequency increases above the grid
self-resonant frequency , the "grounded-grid" behaves progressively
less as though it were grounded and the feedback, or regeneration,
path between the input and the output of the amplifier-tube becomes
more and more conductive to RF current.. This combination is not
desirable unless the designer intends to build an oscillator.

Anti-Parasitic Techniques and Q:
Another important rule is: Q is equal to Reactance divided by
Resistance, or Q= X/R . Q can be decreased by increasing the
resistance, or by decreasing the reactance, or both.

One obvious way to lower Q is to use resistive, or low-Q, conductors.
Silver-plated copper strap has the highest VHF-Q known to science at
room temperature and yet silver-plated copper strap is commonly used
for anode-circuit wiring and for VHF "parasitic-suppressors" in HF
amplifiers. A more accurate name for a silver-plated
parasitic-suppressor would be a parasitic-supporter.

The Q of copper is about 94% of the Q of silver, so copper does not
provide an appreciable improvement in Q reduction over silver. Trying
to build a low-Q circuit with high-Q silver or copper conductors
makes about as much sense as trying to make a pencil eraser out of
Teflon®.

Reducing the inductive reactance by shortening lead lengths may
improve stability IF the shortened lead places the cathode and
anode-circuit self-resonant frequencies farther apart.

Another method of improving stability is to tune out some of the
inductive reactance in the grid structure by bypassing the grid to
the chassis with small capacitors. This increases the self-resonant
frequency of the grid circuit to a point where the amplifier-tube
will have less amplifying and oscillating ability.

The first grounded-grid amplifier that I know of that used this
technique used (4) 811As and was built by the Collins Radio Company.
Many currently produced commercial grounded-grid amplifiers still use
this circuit. I discussed this in a previous article about
parasitic-oscillation in grounded-grid amplifiers. {"Grounded-Grid
Amplifier Parasitics", Ham Radio Magazine, April 1986, page
31.}

Grid-inductance cancelling capacitors are most effective when used
with older design amplifier-tubes like the 811A that have a
considerable amount of internal grid-inductance to cancel. This
technique is only mildly effective at improving amplifier stability
in modern amplifier-tubes, that have inherently low
grid-inductance.

Another anti-parasitic technique that I discussed in the article was
the use of an input parasitic suppressor-resistor, to lower the VHF-Q
at the self-resonant frequency of the input (cathode) circuit. Input
suppressor-resistors also reduce intermodulation distortion (IMD)
with the tradeoff of a slight increase in the drive power requirement
to the amplifier.

Input parasitic-suppressor resistors are moderately effective at
stabilizing unruly amplifiers, but they are not always 100%
successful. After the article about parasitic oscillation was
published, about 5% of the follow-up letters and phone calls I
received were from people who reported that their amplifiers were
more stable with input suppressor-resistors than without, but still
not perfectly free from the foreboding signs of instability like
minor arcing and spitting at the tuning capacitor and/or bandswitch.
The only area left for improvement was the anode-circuit.

In Search Of A Better Anode Parasitic-Suppressor:
The trouble with trying to troubleshoot a parasitic-oscillation
problem is that the crazy things are not always predictable. It may
be that just the right transient or rapid sequence of transients
needs to come along to get the ball rolling. For example, you can
change something like a conductor-length in a marginally stable
amplifier and it will behave for months. When you are beginning to
believe that the problem is "fixed", and you confidently put the rest
of the screws in the cabinet, it will unexpectedly arc or burn-up the
parasitic-suppressor resistor, or worse.

The perfect amplifier to experiment with would be one that had an
unusually high gain amplifier-tube or tubes that consistently
exhibited instability problems even with input suppressors installed.
By a great stroke of good luck, just such an amplifier came into the
possession of NF7S [Ed], who lives in Phoenix, Arizona. From
Ed's point of view it was initially a stroke of bad luck.

The amplifier was a newly purchased model which uses a pair of
3-500Zs with either 2200V (CW) or 3200V (SSB) on the anodes. The new
amplifier made an arcing sound, but he was not concerned since, on
page 14, the instruction manual said that this arcing sound was
"normal". After a few months the "normal arcing" had burned off some
of the contacts on the output section of the bandswitch. The missing
contacts made the amplifier inoperative. This was not an isolated
case because I know of at least eleven other hams who have had to
replace the output bandswitch on the same model amplifier.

Ed contacted factory-service via an authorized dealer and described
the problem. He was told that the output bandswitch was damaged by:
someone who had rapidly switched the bandswitch while transmitting at
full power. He had unpacked the new amplifier himself from a
factory-sealed carton. He knew that he had never hot-switched the
bandswitch. He immediately realized that he was talking to the wrong
people.

I have heard the same outrageous story from other competent amateur
radio operators who had talked to factory-service[?] about
the same problem with this amplifier. I do not believe that any of
these people were stupid enough to try band-switching the amplifier
while transmitting.

He discussed his amplifier problem with me and questioned whether the
voltage capability of the tuning capacitor and the output bandswitch
were adequate for this application. Since the actual breakdown
voltage of these components is above 5000VDC at sea level, and the
maximum RF voltage is only about 2600V-peak, nothing should arc-over
unless the amplifier was operated at an extreme altitude that would
probably cause the operator to pass-out because of anoxia. Clearly,
this was not the case in Phoenix, Arizona.

As the frequency of a specific AC voltage increases, its gas
ionization ability also increases. This effect can be seen in the
manufacturer's voltage versus frequency ratings for RF-rated relays:
The rated RF peak operating voltage always decreases as frequency
increases. This is one of the reasons why the waveguides of
high-power radar transmitters are pressurized with dry nitrogen
gas.

The presence of an unwanted AC voltage, with a frequency that was
much higher than the normal 29.7MHz maximum, was indicated in Ed's
amplifier. The source of this voltage could be a VHF
parasitic-oscillation.

I recommended that Ed install some input suppressor resistors
consisting of a pair of 10 ohm, 2W metal{oxide}film [MOF]
resistors in series with the RF-input connection to the 3-500Z
cathodes. After replacing the original bandswitch and adding the
input suppressor-resistors, he was still noticing arcing in the
general area of the bandswitch.

He threw in the towel. He asked me to see if I could fix the unruly
amplifier; I said I would try. The amplifier and the original,
damaged bandswitch, that he had replaced, made the trip to
California.

The damaged bandswitch revealed that the most severely
burned/vapourized switch parts were the anode tuning capacitor padder
contacts for the 3.5MHz and 1.8MHz positions. The next most-roasted
contacts were for the 28MHz tank-coil tap. The 21MHz tank-coil tap
contacts were burned less than the 28MHz contacts and the 14MHz
contacts were not burned. The pattern was clear: Only the contacts
that were close to the anode were damaged. And the contacts that were
closest to the anode were damaged the most.

The voltage that did this damage had a remarkable ability to jump an
air-gap and also deteriorated very rapidly as it tried to travel
through the inductance of the tank-coil. HF energy would have no
problem traveling through the inductance in the tank-coil. The only
kind of voltage that fits this profile is a high-voltage with a
frequency in the VHF range.

Before operating the amplifier, I installed a 5.1 ohm, 2W MOF
resistor in series with the HV positive lead. The resistor will act
like a HV fuse and current limiter if a full-blown
parasitic-oscillation occurs. This limits the discharge current pulse
from the considerable number of joules of stored energy in the HV
filter capacitor bank. If unlimited, this current pulse can disturb
the grid to filament alignment in the amplifier-tube[s] which
can cause fatal, grid to filament shorts.

A ceramic 10 ohm, 7W to 10W wirewound resistor would provide even
better protection. A higher wattage resistor should be used only if
justified by increased anode-current demand because the resistor is
supposed to burn-out quickly during a circuit-fault and stop the flow
of current. .

As a further precaution before firing-up the amplifier, I checked the
10W cathode bias zener diode. As is often the case after a parasitic
oscillation and its accompanying large current pulse, the zener diode
was found to be shorted. The zener diode was replaced by a series
string of (7) ordinary, perfboard mounted, RF-bypassed, 1A, >50piv
silicon rectifiers with the polarity arrows pointing opposite that of
the original zener. This provides about 5 volts of cathode
bias-voltage during transmit.

{The polarity is opposite because the new diodes will be operated in
the forward conducting (.75v/diode) direction instead of in the
reverse, zener-breakdown direction}

My first encounter with the unruly amplifier exceeded my wildest
expectations. Even with input suppressor-resistors installed, this
amplifier would oscillate reliably with only 2200V on the anodes on
the 14MHz and 28MHz bands! With 3200V applied, the amplifier was
unstable on some additional bands as well. I was impressed. It was an
electronic "Pandora's Box". This amplifier was perfect for
anti-parasitic R and D.

This situation was amazing to me because I owned an identical model
of the same amplifier that had been stabilized by using the same
input suppressor-resistor circuit that was used in the unruly
amplifier. The only difference between the two amplifiers was the
particular pair of 3-500Z tubes.

Ed's 3-500Zs had remarkably high gain. With 100W drive at 3.8MHz,
they would deliver 780v p-p [1520W PEP] into a Bird 50
termination. This does not necessarily mean that they would have also
had abnormally high VHF gain as well, but it is probably a safe
assumption after witnessing their ability to oscillate at VHF.

I set the unruly amplifier aside for a week and discussed the problem
with some of my amplifier-builder friends. After some enlightening
technical discussions and a suggestion to have the amplifier
exorcised , I was ready to proceed.

In every HF amplifier design, there is an unavoidable VHF tuned
circuit formed by the anode to ground capacitance and the total
inductance of the wires or straps between the anode and the output
tuning capacitor. The resonant frequency of this anode-circuit can be
varied only slightly by adjusting the output tuning capacitor. I
measured the anode-circuit's self-resonant frequency in the unruly
amplifier, with a dip-meter coupled to the wire between the HV
blocking capacitor, and the anode-choke. I found a very sharp, high-Q
dip at 130MHz.

Next, I checked the self-resonance of the center-conductor of the
coax that delivers the input signal to the cathodes. The input
circuit self-resonated near the same frequency. This was not
good.

Much of the inductance that formed the resonance in the anode-circuit
appeared to be in the 50mm [2 inches] of "U"-shaped #12
copper wire that connected the HV blocking capacitor to the top of
the anode RF-choke. This innocent looking #12 wire has about 39nH of
inductance. At 130MHz this inductance has a reactance of +j32. I
soldered a 5.1 ohm non-inductive MOF resistor, with "zero"
lead-length, across the "U"-shaped #12 wire to damp the Q of the
tuned circuit. I "fired up" the amplifier on the 14MHz band and
applied drive power. As usual, I saw fire and I heard a familiar
bang. The fuse-resistor exploded again as did the added 5.1 ohm MOF Q
damping resistor ! Thanks to the fuse resistor, the 3-500Zs remained
undamaged and unshorted after this, fifth, full-blown
parasitic-oscillation..

The 5.1 ohm Q-damping resistor's demise was amazing because it was
virtually shorted-out by less than 0.0003 DC ohms of #12 copper wire
when it went kaput ! This resistor had an overload rating of 20W for
5 seconds and it had been destroyed in milliseconds. The only thing
that could have so quickly blown away a tough, essentially DC and HF
shorted resistor like that was VHF current in the multi-ampere
range.

I concluded that the anode-circuit self-resonance of 130MHz was
probably the culprit due to the 3-500Z's 110MHz+ rating and the fact
that the input resonance was tuned to almost the same frequency. If I
could increase the self-resonant frequency of the anode-circuit to a
higher frequency, where the 3-500Z's excellent amplifying ability was
waning, I suspected that it might reduce the chance for a
parasitic-oscillation.

I also decided that, because of the extremely sharp dip at 130MHz,
the high Q of the anode-circuit was probably another contributing
factor. This problem seemed to be exacerbated by the fact that high
VHF-Q silver-plated strap had been used for the combination
anode-suppressors/anode-leads. It did not seem logical to use the
highest Q material to build a circuit that obviously requires a low-Q
to prevent the creation of a transient- induced VHF seed-voltage that
could start a parasitic-oscillation.

Low-Q Conductors:
The obvious choice for a low-Q conductor is nichrome ribbon or wire.
It has 60 times the resistance of copper or silver. Q-measurement
tests on a VHF Q-meter, confirmed that nichrome produces a much lower
Q than any other commonly available conductor material.
Unfortunately, nichrome wire and, especially, flexible nichrome
ribbon, is not easy to find or inexpensive. Soft stainless-steel
makes a good second-choice because it has 10 times the R of copper
and it is commonly available.

Anode-Circuit Modifications:
The #12 copper wire was replaced with a strip of nichrome ribbon
about 3mm in width and 35mm long. A three-turn inductor, with an
inside diameter of about 6mm to 7mm, made from #18 [1mm] soft
stainless-steel wire was connected in parallel with the ribbon in
order to stagger-tune the circuit. This increased the self-resonant
frequency of the anode-circuit to about 150MHz and also lowered its
apparent Q.

It is not possible to connect a VHF Q-meter to the anode-circuit of
an amplifier, but I concluded that the in-circuit VHF-Q had been
reduced appreciably. I arrived at this conclusion by judging how
closely the dipmeter had to be coupled to the anode-circuit to obtain
a 10% meter dip at resonance for each type of conductor material.

The factory-original, silver-plated, high VHF-Q L/R
parasitic-supporters, were replaced with low VHF-Q L/R suppressors
made from two 100, 2W metal{oxide}film [MOF] resistors in
parallel, shunted by a 70nH inductor made from #18 stainless-steel
wire. The inductor has 3-turns. A 9/32" drill-bit shank can be used
as a winding-form. To keep the circuit's VHF-Q as low as possible,
#18 stainless-steel wire was also used for the the leads at the ends
of the anode-suppressor assembly. The ends of the wire leads are bent
into circles for mounting with the original screws.

Construction Notes: 1: The inductor and each MOF resistor should be
parallel to each other and separated by a cooling air gap of about
2mm. Note 2: To avoid a short-circuit and to facilitate cooling, the
inductor must not be wound on top of the resistors because the
conducting part of these resistors is on their outside surface.

For an even lower Q and better parasitic-suppression, the conductors
could be made from nichrome wire in place of the stainless-steel
wire.

If an amplifier shows signs of instability with the 3-turn suppressor
inductors, try 3 1/2 or 4-turn inductors. Caution, the inductance can
not be arbitrarily increased because too-much inductance will cause
the inductor's voltage drop to be too great for the parallel 100, 2W
resistors on the 28MHz band. The reason for this is that, on the
28MHz band, with an anode-voltage of 3KV, there is approximately 1.8a
of RF current-circulating through each 3-500Z anode lead due to the
4.7pF anode to grid (ground) capacitance of each anode.

In amplifiers with longer anode-circuit lead lengths, two or more of
these suppressor assemblies can be connected in series with each
anode lead for an even lower Q.

Results:
The once unruly (TL-922) amplifier has shown no signs of instability
since the anode-circuit was modified with low-Q conductors - even
with all of the screws in the cabinet! The output power appears to be
unchanged on a wattmeter although it is probably about 10 watts lower
at 29MHz as a result of using the low-Q anode-circuit conductors.

The same anti-parasitic technique was used successfully on several
unstable Heathkit SB-220 amplifiers; two, Henry Radio Co. 3CX1200A7
amplifiers and also on a notoriously unstable Viewstar amplifier that
had previously destroyed a pair of 3-500Zs and numerous, other
components as the result of a parasitic-oscillation.

A Closer Look At How And Why A Successful Parasitic-Suppressor
Works:
A successful parasitic-suppressor must perform two, interrelated
tasks. The first task is to reduce the flywheel-effect of a VHF
self-resonant circuit by reducing the Q of that resonant circuit. The
flywheel-effect is essential to oscillation. Reducing the
flywheel-effect will reduce the chance of a parasitic-oscillation.
The second task of a suppressor is to reduce the VHF voltage-gain of
the amplifier stage.

The voltage-gain of an amplifier is approximately proportional to the
output load-resistance (RL) placed on the amplifier-tube. High RL
means high voltage-gain and low RL means low voltage-gain. If the VHF
voltage-gain of an amplifier-tube can be made low enough, by
decreasing the VHF RL , the VHF voltage-gain of the amplifier will be
so low that it can not oscillate at VHF. If a high-Q
conductor-inductor is used to connect the anode of the amplifier-tube
to the, essentially VHF-grounded, tuning capacitor, a high-Q
parallel-resonant-circuit will be formed. The capacitance in this
parallel-resonant-circuit is the output capacitance of the tube and
the inductance is the built-in inductance in the leads between the
anode-connection [plate-cap] and the tuning-capacitor. A
high-Q parallel resonant circuit acts like a very high resistance at
its resonant frequency. Thus, the amplifier has a very-high RL and a
very-high voltage-gain at the VHF resonant frequency which greatly
increases the risk of a VHF parasitic-oscillation. SeeFigure 1,C.

A low-Q, parallel-resonant circuit will have a relatively
low-resistance at its resonant frequency. If two, low-Q, paralleled,
conductor/inductors of slightly different inductance are connected in
parallel and to the same capacitor (Cout) a dual resonant, broadband
effect and an even lower-Q will result. This is similar to the
broadbanding-effect that is achieved when the primary and secondary
of an IF-transformer are tuned to different frequencies. This
technique lowers the VHF-Q even further and decreases the VHF output
RL which further decreases the VHF voltage-gain of the amplifier. The
goal of parasitic-suppression is to reduce the net (VHF)
voltage-gain, by lowering the VHF-Q, which lowers the VHF load
resistance on the amplifier-tube, so that the amplifier-tube can not
oscillate.

In a typical parasitic-suppressor, the two, low-Q paralleled
conductor-inductors are: the suppressor's resistor, which makes the
lower-inductance current path, and the nichrome inductor, which makes
the higher-inductance current path. Both of the inductances in a
parasitic-suppressor can also be constructed solely out of low-Q wire
or ribbon as was the case for the low-Q replacement for the #12
copper buswire in the TL-922.

The "Bottom Line":
High-Q conductors, such as silver and copper, are the best choice for
the anode-circuit/tank-circuit conductors in a VHF amplifier or VHF
oscillator.

Copper is the best material for the conductors in a HF tank- circuit
or tuned-input circuit. Silver-plating the copper will improve the
appearance but not the performance at HF.

Nichrome exhibits a very low VHF-Q. Thus, it is a suitable material
to use for anode-circuit, input-lead and suppressor conductors in an
HF -amplifier. Round conductors exhibit a lower VHF-Q than flat
conductors due to skin effect.

Appropriate Conductor Sizes:
1/4 inch [6.35mm] nichrome ribbon conductor is satisfactory
for anode-circuits carrying up to about 8A of RF circulating-current.
The circulating-current through the anode-lead of a typical 1500W
amplifier is usually much less than this. The conductor width should
be held to a minimum to lower the VHF-Q for better stability. It
would not be good engineering practice to use 1/4 inch nichrome
ribbon if a smaller conductor will carry the current. Bigger or wider
conductors are not appropriate unless a smaller conductor is
overheating from the RF circulating-current during 10 meter band
operation.

The safe RF current carrying capacity of #18 gauge nichrome wire, in
free-air, is probably about 3 amperes at 30MHz.

Construction Tips:
Nichrome and stainless-steel can be easily soldered with an ordinary
soldering iron by using a special flux that is made for soldering
nickel-chromium alloys and 430ºF tin-silver solder. These
materials are sold in hobby shops and in welding-supply stores.

Notes:

There is no single "sure-cure" for every case of amplifier
instability.

1. Taming especially unruly amplifiers may require the intelligent
use of a dip-meter, several anti-parasitic techniques, more than one
L/R parasitic-suppressor per anode-lead and a, VHF Q-lowering, 1
metalfilm [MF] resistor in series with the L/R
parasitic-suppressor.

2. In some cases, it may help to add a low-Q, series-resonant L/R/C
suppressor between the cathode and ground. The resonant frequency of
this series circuit should be at, or slightly higher than, the
self-resonant frequency of the anode-circuit. The resistor should be
a 1 to 5, 2W MOF or MF-type and the capacitor is 25pF. The inductance
is controlled by adjusting the leadlengths on the resistor and the
capacitor. The resonant frequency of this circuit is difficult to
check because the cathode must be directly shorted to ground and the
resistor must be bypassed with a straight wire in order to find the
dip on a dipmeter.

In rare cases, a VHF self-resonance in the anode HV RF-choke or in
the filament-choke can become a player in a parasitic-oscillation.
This problem can be overcome in these ways: A filament-choke can be
effectively isolated by placing a VHF attenuator-rated ferrite-bead
(Mu850) over each filament lead on the filament side of the
filament-choke. An anode HV RF-choke can be effectively isolated by
placing an unbypassed 10, 15W, wirewound resistor in series with
either end of the choke.

Parasitic oscillation can be one of the most vexing amplifier
problems. If you would like to discuss any part of this article or
the malady in general, please feel free to call me at [805]
386-3734.

Fixing a parasitic oscillation problem is definitely different. In
the end, the only reward you get is: no surprises. Just be sure that
you put all the screws in the cabinet before you relax.